Los Angeles shut down 1,000 public schools on Tuesday, after an email threat that claimed to be from "an extremist Muslim who has teamed up with local jihadists" according to a congressman.
The schools were closed for the day and searched after the anonymous email, but authorities in another US city said they got the same threat - and quickly concluded that it was a hoax.
The email to Los Angeles school boards raised fears of another attack like the deadly shootings in nearby San Bernardino, but police in New York City on the other side of the country said they received the same threat and dismissed it.
New York Mayor Bill de Blasio told reporters that he was "absolutely convinced" there was no danger to schoolchildren in his city, the Associated Press reported.
"There was nothing credible about the threat. It was so outlandish," de Blasio said.
New York Police Commissioner William Bratton agreed, saying that it looked like the sender of the threat watched a lot of the terrorism drama Homeland.
Hours later, a member of the US House Intelligence Committee also said the threat was believed to be a hoax.
Los Angeles officials defended the move.
"It is very easy in hindsight to criticice a decision based on results the decider could never have known," LA Police Chief Charlie Beck said.
Los Angeles Congressman Brad Sherman said the email author claimed to be an extremist Muslim who had "teamed up with local jihadists".
He added: "The text of the email does not demonstrate that the author has studied Islam or has any particular understanding of Islam," according to The LA Times.
The email also mentioned explosive devices, assault rifles and pistols, LA Times said, and police sources said it was traced to an IP address in Frankfurt.
The shutdown abruptly closed more than 1,187 schools attended by 640,000 students across Los Angeles.
Superintendent Ramon Cortines said every campus would be searched, and he asked for a report on the searches certifying that all buildings are safe.
The threat, the New York officials said, came in the form of a "generic" email to many cities around the country. In New York, it was received by a superintendent early Tuesday.
Bratton called the closure a "significant overreaction."
"We cannot allow ourselves to raise levels of fear," said Bratton, who once ran the Los Angeles Police Department.
The person who wrote the note, Bratton said, claimed to be a jihadist but made errors that indicated the writer was really a prankster, including spelling the word "Allah" with a lowercase "a."
White House spokesman Josh Earnest said he would not second-guess the decisions made in Los Angeles or New York. He said the FBI has been in touch with California authorities.
The decision to close the district disrupted the morning routines of many Los Angeles families.
Lupita Vela, who has a daughter in the third grade and a son who is a high school senior, called the threat "absolutely terrifying" in light of the San Bernardino attack, which killed 14 people earlier this month.
She got an automated phone call informing her of the closure.
"I know the kids are anxious," she said.
District spokeswoman Shannon Haber said the threat was sent by email to a school board member and was believed to have come from an IP address in Frankfurt, Germany.
Beck said the email was specific to all the campuses in the district and included implied threats about explosive devices, assault rifles and machine pistols.
The city schools commonly get threats, but Cortines called this one rare.
"It was not to one school, two schools or three schools," he said at a news conference. "It was many schools, not specifically identified. But there were many schools. That's the reason I took the action that I did."
The San Bernardino attack influenced the decision to close the entire district, Cortines said.
The superintendent said the district police chief informed him about the threat shortly after 5 a.m.
"He shared with me that some of the details talked about backpacks, talked about other packages," Cortines said.
Vela said she worries about talking to her kids about the threat and terrorism in general. She's concerned about her daughter feeling safe in class.
"I don't want this to be in the back of her head," she said. "Who knows what it does psychologically to kids? Is this going to cause her some kind of trauma so that she's not going to feel safe at school?"
The closure came the same day classes were canceled at San Bernardino Valley College because of a bomb threat.